Electrostatic photographic printing



Aug. 14, 1956 M. L, SUGARMAN, JR 753,524

ELECTROSTATIC PHOTOGRAPHIC PRINTING Filed Dec. 30, 1953 ifi NENTOR.

United States Patent O s Claims. (c1. 9s-1.s)

Princeton,

N. J., assignor to of America,

a corporation of Dela- This invention relates to improved methods and means of electrostatic printing.

An electrostatic printing process is that type of process for producing a visible record, reproduction or copy which includes as an intermediate step, converting a light image or electrical signal into an electrostatic charge pattern on an insulating base. The process may also include the conversion of the charge pattern into a visible image which may be a substantially faithful reproduction of an original except that it may be a different size.

A typical electrostatic printing process may include coating a surface of a conductive backing plate with a photoconductive insulating material such as selenium, anthracene, or sulphur, and then providing an over-all electrostatic charge on the surface of the photoconductive material. An optical image is focused on the charged surface, discharging the portions irradiated by the light rays, while leaving the remainder of the surface in a charged condition, thereby forming an electrostatic image. The e.ectrostatic image is rendered visible by applying a developer powder which is held electrostatically to charged areas of the sheet. The powder image thus formed may be Xed directly to the photoconductive coating or it may be transferred to another surface and then fixed thereon. A detailed description of the steps of this process may be found in U. S. Patent 2,297,691 issued October 6, 1942, to C. F. Carlson.

In the process above described, the steps or" electrostatic image formation and subsequent development of the electrostatic image are separate and distinct operations. For example, the electrostatic image is formed utilizing apparatus for giving the photoconductive surface an overall charge and apparatus for projecting an optical image on the charged surface. When the electrostatic image has been formed, additional apparatus is employed to render the electrostatic image visible.

Because these steps are separate and distinct operations, the photoconductive layer must have the ability to store the electrostatic image at least for the period of time necessary to form and to develop the electrostatic image. By combining the two steps in a single operation, photoconductive materials having very shor storage times, that were hitherto impractical, may be used. It is also commercially desirable to combine these steps into a single operation in order to speed up the printing process and simplify the required apparatus.

An object of this invention is to provide improved methods and means of electrostatic printing.

Another object is to provide a single, simplified electrostatic printing apparatus for accomplishing the steps of forming and rendering an electrostatic image visible.

Another object is to provide methods and means of electrostatic printing that may use photoconductive materials having relatively short storage times.

In accordance with the present invention a visible powder image may be produced by first placing a thin layer of developer powder upon the upper surface of a 2 photoconductive insulating layer upon a lower electrode and spaced from an upper electrode, then simultaneously applying a unidirectional electric field to said developer powder layer and projecting a light image upon said photoconductive insulating layer. Simultaneously application of the electric field and projection of a light image charges the developer powder particles lying upon the illuminated areas of the photoconductive insulating layer, attracts said charged particles away from said photoconductive insulating layer unto said upper electrode, particles by contact with said upper electrode and attracts said discharged particles to said photoconductive insulating layer. The electric field is maintained upon said developer powder layer and the light image is maintained upon said photoconductive insulating layer to repeat said charging and discharging steps until the lateral movement accompanying said oscillatory movement deposits the oscillating powder particles in the unilluminated areas of the photoconductive insulating layer leaving a visible powder image on the photoconductive insulating layer.

The invention will be more easily and fully understood from the following detailed description in conjunction with the accompanying drawing in which:

Figure l is a partially schematic sectional elevational View of a camera embodying this invention, and

Figure 2 is a sectional elevational view of the electrophotographic pack that may be used in the camera of Figure l.

Similar reference characters are applied to similar elements throughout the drawing.

Referring to Figures l and 2, an electrophotographic pack 79 comprises an upper brass electrode 27 spaced about 1/s" above a lower electrode by means of spacers 67. The lower electrode comprises a transparent back ing plate 21, such as glass, a transparent electrically conducting layer 23, such as a NESA coating marketed by the Pittsburgh Plate Glass Co., Pittsburgh, Pa., superimposed thereon, and a coating of a photoconductive insulating material 25 such as selenium superimposed upon the transparent coating 23. A thin even layer of developer powder 41 such as magnesium silicate is placed on the upper surface of the photoconductive layer 25. The powder may be applied, for example, by sprinkling the powder from a suitable shaker before the electrophotographric pack 79 is assembled. A camera is provided comprising a lens 81, a lens holder S3, a shutter 84, a shutter mechanism 8S, an enclosure 37, a pack holder 59 and a spring clip i. The electrophotographic pack 79 is mounted horizontally in the upper rear portion of the camera by means of a hinged door 93 and held in place by means of a pack holder 89 and a spring clip 9i. The upper electrode 27 and the transparent coating 23 of the lower electrode are connected to a voltage source Si through double-pole, double-throw switch 63 and a potentiometer 65. A mirror 73 support ed upon mirror holders is mounted diagonally at the back of the enclosure S7 such that an optical image passing horizontally is directed upwards to the electrophotographic pack 79.

A negative voltage of about 60G volts is applied to the upper electrode Z7 by throwing the switch 63 and adjusting the potentiometer 65. The shutter S4 is opened by means of shutter mechanism S5 allowing the image of an object 99 to fall upon the photoconductive coating 23 of the lower electrode. The electrical resistance of the illuminated area of the photoconductive coating 23 is reduced due to the presence of light, allowing an electric charge to move to the upper surface of the photoconductive coating 23, thus forming an electrostatic image corresponding to the illuminated areas. The developer powder 41 resting on the charged areas of the photoconf 631 AGreenwich Street, New

detve coating '23 becomes 'charged in the same polarity as the 4area uponwhich 'it is resting. The 'powder thus charged is attracted upwards due to the electric eld present including the relatively strong ield existing between 'the upper and lower electrodes. When the charged powder particles 41rea`ch 'theupper'electrode 27, they -are discharged and return to the lower electrode and Vthis process "is repeated rapidly. The powder particles resting inthe charged areas of the electrostatic image are made to 'oscillate between the electrodes in this manner. During lthe oscillations -'the particles have Va certain amount of lateral movement, which causesithem to drift away from the'charged areas ofthe electrostatic image. The particles 41'finally come to 're's't in the non-charged areas ofthe electrostatic image, 'tlierebyprodueing a powder image. The eleet'ric iieldis 'dises'tablis'h'edby opening the switch 63`and`flie electrophofogiraphi'c pack is removed -from the camera. rThe visible .powder image maybe xed to the surface upon which it'rests or it may be transferred to another 'surface and VYfixed thereon by any of the conventional methods. For example, an adhesive may be sprayed on the powder image, or, if thepowder is fusible, it may be fused to the surface upon which it rests by the "application of heat.

Almost any photoconductive material may be used. Examples are Va VVsingle crystal or -an evaporated layer of antimony trisulphide, photoconductive selenium, and lead iodide. Mixtures of 'a photoconductor and up to 90% "of a'resin schas a silicone-resin may also be used. Many electrostatic'p'rinting'processes require the use of photoconductive materials having relatively `long relaxation times. The relaxation time of a photoconductor is a measure of its ability to store an electric charge and is commonly expressed as theperiod of time in which an arbitrary per cent of acharg'e stored on its surface will leak off. Unlike 'these processes, the photoconductive` 'materials that may be used in the ,process of this invention are not limited by their relaxation times because the lphotoconductive material yis not required to store a charge. Thus materials may be used that are ordinarily impractical. The photoconductive material must 'be suiciently insulating to prevent a build up of charge Aon its surface in the darkness while'the voltage is applied to the electrodes. The switch 63 may be synchronized with the Vshutter `84 so that the electric eld is 'established for only a fraction of a minute and therefore photoconductive materials that are sometimes considered conducting may be used.

YWhile any type electromagnetic radiation may be projected upon the photoconductive layer, the spectral range of the radiation must be within the range of sensitivity of the photoconductive material. Thus the choice of photoconductive material and radiation are dependent upon one another.

Any powder may be -usedto produce the visible image by the method of this invention. Materials that have a relatively Vhigh electrical conductivity became charged and attracted upward more rapidly. However, relatively vinsulating powders 'work equally well although longer exposures are required.VV It is preferred to use powders'with particle size ranges between l0 and 100 microns. Surface forces interfere with theprocess when the powder is too line. Particles that are too coarse are difficult to handle and tend to lose delinition in the image. ExamplesV of""suitable developer powders are sawdust, carbon black, brass,`ma'gne`sium' silicate and synthetic resins such as Cibanit, an aniline-formaldehyde condensationV product marketed by the Ciba 'Chemical Co., York, N YY. The powders may be stained with dyes or mixed with pigments.

The spacings of the-A electrodes and the lapplied voltage are notat all critical. Itis only .necessary` to provide an electric iield which whenA added to the smaller eldof the electrostatic image will lcause vthe charged .powder to movefupwards. -Electric'elds of the order of 4to- 8 are preferred for the specic arrangement of the example. The preferred range of iield strengths in other cases will be determined in part by the arrangement of the elements and the nature of the materials used. Y

There have thus been described improved methods and means of electrostatic printing. The apparatus is simple in construction and rapid in operation. Photoconductive materials having relatively short storage times that were hitherto unfeasible may Ynow be used.

What is claimed is:

l. A method of electrostatic printing comprising rst placing a thin layer of developer powder upon the ,upper surface of a photoconductive insulating layer upon a lower electrode :and spaced from .an upper electrode, then simultaneously producing a unidirectional electric tield between said electrodes and projecting a light image upon said photoconductive insulating layer to charge the developer powder particles lying upon the illuminated areas of the photoconductive insulating layer, attract said charged particles away from said photoconductive insulating layer unto said upper electrode, discharge said charged particles by contact with said upper electrode and kilovolts per inch attract said discharged particles to said photoconductive panying said oscillatory movement -upper surface of a insulating layer, and maintaining said electric tield between said electrodes and said light image upon said photoconductive insulating layer to repeat said charging and discharging steps until the lateral movement accomdeposits the oscillating powder particles in the unilluminated areas of the photoconductive insulating layer leaving a visible powder image on the photoconductive insulating layer.

2. A method according to claim l including xing the powder image.

3. A method ing a thin layer of of electrostatic printing comprising placmagnesium silicate powder upon the photoconductive insulating selenium layer upon a lower electrode and spaced from an upper electrode, then simultaneously ,producing a unidirectional electric eld of the order of 4,000 to 8,000 volts per inch betweenV said electrodes and projecting a light image upon said photoconductive .insulatingflayer to charge the magnesium silicate powder particles lying upon n the illuminated areas of the photoconductive insulating layer, attract said charged particles away `from said photoconductive insulating layer unto said upper electrode, `discharge said charged particles by contact with said upper electrode and attract said discharged particles to said photoconductive insulating layer,.and maintaining said electric iield between said electrodes and said light image upon said photoconductive'insulating layer to repeat said charging and discharging steps until the lateral movement Vaccompanying said oscillatory movement deposits the oscillating Vmagnesium silicate lpowder parti-V clesin the unillurninated areas vof the Aphotoconductive insulatinglayer leaving a visible .powder image on the photoconductive insulating selenium layer.

References Cited in the file of thispatent OTHER REFERENCES New Developments -in Xeroradiography-Non-De structive Testing; Summer 1951. Vol. l0, No. l-pages 825; page 1S .particularly relied upon. (Photostat copy in Div. 67.) 

1. A METHOD OF ELECTROSTATIC PRINTING COMPRISING FIRST PLACING A THIN LAYER OF DEVELOPER POWDER UPON THE UPPER SURFACE OF A PHOTOCONDUCTIVE INSULATING LAYER UPON A LOWER ELECTRODE AND SPACED FROM AN UPPER ELECTRODE, THEN SIMULTANEOUSLY PRODUCING A UNIDIRECTIONAL ELECTRIC FIELD BETWEEN SAID ELECTRODES AND PROJECTING A LIGHT IMAGE UPON SAID PHOTOCONDUCTIVE INSULATING LAYER TO CHARGE THE DEVELOPER POWDER PARTICLES LYING UPON THE ILLUMINATED AREAS OF THE PHOTOCONDUCTIVE INSULATING LAYER, ATTRACT SAID CHARGED PARTICLES AWAY FROM SAID PHOTOCONDUCTIVE INSULATING LAYER UNTO SAID UPPER ELECTRODE, DISCHARGE SAID CHARGES PARTICLES BY CONTACT WITH SAID UPPER ELECTRODE AND ATTRACT SAID DISCHARGED PARTICLES TO SAID PHOTOCONDUCTIVE INSULATING LAYER, AND MAINTAINING SAID ELECTRIC FIELD BETWEEN SAID ELECTRODES AND SAID LIGHT IMAGE UPON SAID PHOTOCONDUCTIVE INSULATING LAYER TO REPEAT SAID CHARGING AND DISCHARGING STEPS UNTIL THE LATERAL MOVEMENT ACCOMPANYING SAID OSCILLATORY MOVEMENT DEPOSITS AND OSCILLATING POWDER PARTICLES IN THE UNILLUMINATED AREAS OF THE PHOTOCONDUCTIVE INSULATING LAYER LEAVING A VISIBLE POWDER IMAGE ON THE PHOTOCONDUCTIVE INSULATING LAYER. 